The production of polymers or copolymers of lower .alpha.-olefins, particularly ethylene and propylene, has gained substantial commercial acceptance. The polymeric products are inexpensive and exhibit a number of commercially useful properties. In the case of the polymerization of ethylene, the process is relatively uncomplicated in that the product type is not influenced by the manner in which the ethylene molecules add to the growing polymeric chain and the polymeric product does not exist in stereoisomeric forms.
In the case of the polymerization of propylene, however, the presence of pendant methyl groups on the polymeric chain provides the possibility of several types of product depending upon the steric regularity with which the propylene units add to the growing polymer chain. Much, if not most, of the commercial polypropylene is crystalline and results from the stereoregular addition of propylene units in a regular head-to-tail manner. The polypropylene in which the addition of propylene units is random is termed atactic. This amorphous form is less desirable and, if present in significant quantities, must be removed as by extraction in order to obtain a more desirable crystalline product.
Also significant from a commercial standpoint is the activity of the polymerization catalyst. A number of the early polymerization catalysts, e.g., trivalent titanium, chromium or vanadium catalysts, were of relatively low activity and the polyolefin product contained a significant proportion of catalyst residues. The removal of such residues as by a deashing step was required in order to obtain a product of commercially acceptable properties. The more recent olefin polymerization catalysts are stereoregular and of sufficient catalytic activity so that extraction and/or deashing steps are not required.
In the terms now conventionally employed for the components, the high activity olefin polymerization catalysts are formed from a procatalyst which typically contains magnesium, titanium and halide moieties as well as an electron donor, a cocatalyst which is usually an organoaluminum compound and a selectivity control agent which may be provided as a partial or total complex with the cocatalyst. Although each of the components has a considerable influence on the polymerization catalyst and process and the polymer product obtained therefrom, the nature of the catalyst as well as the polymerization product seems to be most influenced by the procatalyst. Much of the research directed toward improvement of the olefin polymerization process has been directed toward improvement of the procatalyst component.
Kioka et al, U.S. Pat. No. 4,300,649, describe a solid catalyst component (procatalyst) obtained by heating a soluble magnesium compound such as magnesium chloride with an alkanol of at least 4 carbon atoms in the presence of an ester to produce a solution which is added to titanium tetrachloride and an electron donor to form a procatalyst. Band, U.S. Pat. No. 4,472,521, reacts a magnesium alkoxide with a titanium alkoxide of 4 or more carbon atoms in each alkoxide moiety in the presence of aromatic hydrocarbon. Titanium tetrachloride and an electron donor are added to the resulting solution to form a procatalyst which is post-treated with transition metal halide. Arzoumanidis et al, U.S. Pat. No. 4,540,679, produce a catalyst component by contacting a suspension of magnesium ethoxide in ethanol with carbon dioxide. The addition of an organoaluminum compound in hydrocarbon solvent to the resulting solution produces spherical particles which are employed as a support for titanium moieties upon treatment with titanium tetrachloride. Nestlerode et al, U.S. Pat. No. 4,728,705, solubilize magnesium ethoxide in ethanol with carbon dioxide and spray dry the resulting solution or alternatively use the solution to impregnate catalyst particles. The particles resulting from either modification are useful in the production of a procatalyst of desirable morphology.
A somewhat different type of catalyst component precursor is described by Job, U.S. Pat. No. 4,710,482, wherein a crystalline magnesium compound of the general formula EQU Mg.sub.4 (OR).sub.6 (ROH).sub.10 A (I)
is formed in which R independently is lower alkyl of up to 4 carbon atoms inclusive and A is one or more anions including halide anions or anions of polyhydric phenols having a total oxidation state of -2. This insoluble complex magnesium compound is contacted with a tetravalent titanium halide, a halohydrocarbon and an electron donor to form an olefin polymerization procatalyst. Such complex magnesium compounds are precipitated from solutions in methanol and are converted to procatalysts and then to olefin polymerization catalysts by largely conventional technology. The catalysts so produced are good high activity polymerization catalysts, particularly for the polymerization or copolymerization of propylene. It would be of advantage, however, to provide improved olefin polymerization catalysts.